For anodic oxidation in electrochemical processes, because of its chemical properties, it is preferred to use platinum as anode material. Frequently, it is even the only metal which can be used for such processes.
Although platinum is very expensive, in the case of the electrochemical production of inorganic peroxy acids and of the salts thereof on a large scale, hitherto only massive platinum material has been used. It has, namely, been ascertained that even small amounts of alloying components, such as are used for the improvement of the mechanical strength of the platinum, for example of only 1% of iridium, reduce the current yield of the electrodimerization on the anode. The differing adsorption or desorption behavior of the metals for the anions or radicals and peroxy compounds on the anode surface are held to be responsible for this loss of energy. Also for the production of perhalides, especially of perchlorates and perchloric acid, there is also preferably used platinum since this, in comparison with other materials, for example graphite coated with lead dioxide, has a greater stability and thus a longer life.
Therefore, there is a need for composite electrodes of a base metal with a firmly adhering platinum covering. Composite electrodes are known in which the anode material platinum is fixed as a relatively thin covering on to a corrosion-resistant carrier material which has as good an electrical conductivity as possible. Thus, for example, it is known to produce a platinum covering by cathodic deposition from galvanic platinum baths or platinum salt melts. However, it has been shown that in such a composite material with a platinum layer applied galvanically on to a carrier material, for example on to titanium, the covering does not adhere sufficiently well to the carrier material when it is used as anode for electrolysis. Thus, when using such a composite electrode for the production of peroxydisulphates, only an insufficient period of use can be achieved.
It is also known to produce coatings of platinum by the thermal decomposition of platinum compounds. However, composite electrodes produced in this way only give low yields of peroxydisulphates or perchlorates. This applies especially to platinum oxide/mixed oxide coverings produced in this manner, such as are used for the electrolysis of alkali metal chlorides or for chlorate electrolysis.
Furthermore, all such thermally or galvanically produced platinum coverings for the anodic oxidation of inorganic and organic compounds, for example for the electrolytic production of peroxydisulphates or perchlorates, are too thin since, during the electrolysis, they undergo a wearing away which is so great that it amounts to one gram of platinum per ton of product. In large-scale plants, there is reckoned with a layer thickness loss of up to 5 .mu.m. of platinum per year. The result of this is that, depending upon the nature of the electrolysis and of the technical carrying out thereof, massive platinum with a thickness of up to 100 .mu.m. is employed.
The massive platinum used for the above-mentioned anodic processes is employed, for example, in the form of wires with a thickness of 120 to 150 .mu.m. or as a rolled foil with a thickness of 10 to 100 .mu.m. The electric current is thereby preferably passed through carrier metals on the platinum metal which are anodically stable in the electrolytes in question or are able to form a passive layer, i.e. so-called valve metals. The platinum itself is thereby fixed on to such carrier metals by means of various measures. Titanium, tantalum or zirconium is usually employed as carrier metal.
From Federal Republic of Germany Pat. No. 16 71 425, it is, for example, known firmly to screw a 50 .mu.m. thick platinum foil on to a cylindrical hollow body by mechanical pressing on devices with a high local contact pressure, the substrate being titanium. However, in a composite produced in this manner, the current transfer from the titanium hollow body to the platinum foil takes place exclusively at those points on which the body and the foil are connected with one another by bearing pressure bands and rings. Since an oxidized titanium surface does not conduct current and thus represents an insulating layer, the transfer of the current to the electrochemically effective surface of the platinum only takes place through the thin cross-section of the platinum foil. The result of this is that this must be the thicker, the higher is the current density employed. In the case of continuous operation, such an electrode has a life time of up to 3 years. If the contact resistance between the titanium and the platinum foil increases too much, then the two parts must first be dismantled and the original state must be produced again by mechanical measures. However, this is no longer possible when, due to too high contact resistances, an oxidizing welding of the two parts has taken place in the electrolyte, which is very often the case.
A further problem lies in the fact that, due to frequent electrical flashovers which result from the poor current transfer from the anode tube to the platinum foil, not only the anode tube but also the platinum foil are increasingly more damaged with increasing period of use. Thus, under unfavorable conditions, the platinum foil of a tubular wound anode, such as is described, for example, in Federal Republic of Germany Pat. No. 16 71 425, can, due to a spark discharge to the underlying titanium hollow body, lift off or burn through locally. This leads to a subsequent short circuiting to the cathode surface which is only 3 to 6 mm. away and brings about a destruction of the cell. In extreme cases, this can lead to leakage of the whole electrolysis plant and even to the explosion of partial regions of the electrolyte pipe system.
It is also known to use for anodic electrochemical processes a tantalum-covered silver wire with a diameter of 1 to 2 mm. on which a long platinum wire is fixed spirally by point welding. In the case of another type of anode, on a titanium rod are fixed, by clamping or welding, platinum wires with projecting spokes on both sides, a planar anode covered with platinum wire thereby being formed.
However, all these composite electrodes have the disadvantage that the current passage from the carrier to the active electrode is poor, as a result of which the high current-loaded points of contact heat up and an increased corrosion thereby takes place at these points which, in turn, leads to an impairment of the conductivity and thus to a further heating up.
It is also known to fix a platinum foil to a carrier metal, for example tantalum or titanium, by roll seam welding. This is in part carried out by overlapping placing next to one another of welding points. However, in the case of such a welding process, in order to prevent the burning through of the foil in the case of the welding, the thickness of the platinum foil and of the carrier metal must be of the same order of magnitude. Thus, for example, for this purpose, a 40 .mu.m. thick platinum foil must be used on 50 to 100 .mu.m. thick tantalum. According to Federal Republic of Germany Pat. No.29 14 763, an improvement of the bonding is achieved by roll seam welding of a titanium sheet of 1 mm. thickness with a 10 .mu.m. thick platinum foil and a stainless steel foil of 100 .mu.m. thickness placed thereover, the stainless steel foil subsequently being removed again by chemically dissolving with an acid.
However, in such a welding process, the metallic and thus electrically-conducting connection is only guaranteed on the welding points. At the points not welded with one another, the platinum foil only lies on the carrier metal. The current transfer is there prevented so that a welded composite electrode of this type also displays the previously described disadvantages. Furthermore, these welding points are subjected to strong corrosion if the platinum foil is damaged and this can then result in a direct contact with the electrolyte.
However, the previously described disadvantages can be overcome by a planar contact between the platinum foil and the carrier metal. Thus, for example, it is known to apply an approximately 50 .mu.m. thick platinum foil to a 2 mm. thick, pre-treated titanium sheet by roller plating. However, this process is expensive and, in addition, does not provide a dependable bonding since the metals do not attach to one another equally strongly at all points. Therefore, in the case of the use of such a material in electrolysis, it continuously happens that the platinum covering lifts off in places, a short circuiting to the counterelectrode thereby taking place.
Another possibility of forming a planar bonding between platinum foil and the carrier metal substrate consists in explosion plating. However, this has the disadvantage that a strong distortion, a considerable loss of material in the region near the edge and a fold or wave formation of the platinum foil must be taken into account, this laborious process thereby giving rise to further technical disadvantages. In addition, it is uneconomical.
Finally, it is also known to produce a planar bonding between a platinum foil and a carrier metal substrate by gas pressure diffusion welding (Ch. Nissel in Powder Metallurgy International, Vol. 16, No. 3, p. 13/1984). By hot isostatic pressing (HIP), there is thereby produced a firm mechanical bond between the two metals. However, it has been shown that only on small samples with a surface of a few cm.sup.2 can a metal bonding be obtained which displays satisfactory results in the case of chlorine and chlorate electrolysis. Furthermore, the individual experimental results with regard to strength of adhesion and electrolysis properties are not reproducible. In particular, it has been shown that the cell voltage was different in all experiments. In the case of the production of peroxydisulphates, electrolysis current yields of 0 to 25% were measured with such composite metals.